The objectives of this research are to determine 1) the agreement of stereotactic fine needle biopsy (SFNA) and stereotactic core needle biopsy (SCNB) results with surgical results and 2) the appropriate use of mammographically guided stereotactic fine needle aspiration and stereotactic core needle biopsy, in the diagnosis of breast cancer in women with nonpalpable breast lesions. A randomized clinical trial will be carried out through a consortium of 12 mammography centers nationwide, each contributing 100-400 cases over the grant period. The breadth of mammography practice is well represented among these groups. This offers the opportunity to cover the spectrum of experience, equipment and patient populations, using an agreed upon protocol to evaluate the use of SFNA and SCNB. At all sites, women having had the appropriate mammographic evaluation, and meeting the study entry criteria will be randomized either to the SFNA/SCNB arm or the SCNB arm of the trial. Those in the SFNA/ SCNB arm will have an SFNA followed by a SCNB, then will go on to open biopsy or follow-up. Those in the SCNB will have the SCNB followed by open surgical biopsy or follow-up. Within the SFNA arm, radiologists will also be testing the sampling techniques with and without use of coaxial needle placement on alternate patients. All pathologic and cytologic specimens will be sent to independent experts for a second interpretation, and all mammograms will have second independent readings for characterization of lesions. A decision and cost effectiveness model will be developed and analyzed. Data analysis will consist of estimating agreement, using a kappa statistic, and logistic regression, and estimation of detection of cancer accuracy using a logistic regression analysis. In addition analysis of observer variability, insufficient sample rates, and predictive ability of specific mammographic characteristics will be conducted.